Transient Fluorescence Spectroscopy Research Articles

A Polymethionine Nanoparticle Fluorescent Probe for Sensitive Detection of Naringin and Naringenin.

In this work, we demonstrated a novel, sensitive and effective fluorescent naringin (NRG) and naringenin (NRGe) detection method using polymethionine nanoparticles (PMNPs) as a fluorescent nanoprobe. The PMNPs were first synthesized by autopolymerization of methionine at 90 °C when trace copper ions existed. The as-prepared PMNPs were thoroughly characterized by transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), gel permeation chromatograph (GPC), nuclear magnetic resonance spectroscopy (NMR), transient and steady-state fluorescence and UV-Vis absorption spectroscopy. The quenching mechanism was attributed to the inner filter effect (IFE). Moreover, the developed assay was used successfully to detect NRG and NRGe in real samples of citrus fruits, illustrating that this detection method has great potential application in the field of citrus fruits analysis.

Unraveling the excited-state vibrational cooling dynamics of chlorophyll-a using femtosecond broadband fluorescence spectroscopy.

Understanding the dynamics of excited-state vibrational energy relaxation in photosynthetic pigments is crucial for elucidating the mechanisms underlying energy transfer processes in light-harvesting complexes. Utilizing advanced femtosecond broadband transient fluorescence (TF) spectroscopy, we explored the excited-state vibrational dynamics of Chlorophyll-a (Chl-a) both in solution and within the light-harvesting complex II (LHCII). We discovered a vibrational cooling (VC) process occurring over ∼6ps in Chl-a in ethanol solution following Soret band excitation, marked by a notable ultrafast TF blueshift and spectral narrowing. This VC process, crucial for regulating the vibronic lifetimes, was further elucidated through the direct observation of the population dynamics of higher vibrational states within the Qy electronic state. Notably, Chl-a within LHCII demonstrated significantly faster VC dynamics, unfolding within a few hundred femtoseconds and aligning with the ultrafast energy transfer processes observed within the complex. Our findings shed light on the complex interaction between electronic and vibrational states in photosynthetic pigments, underscoring the pivotal role of vibrational dynamics in enabling efficient energy transfer within light-harvesting complexes.

Photocatalytic dye degradation by synergistically Cu LSPR and Schottky barrier enhanced Cu/g[sbnd]C3N4

The issue of water pollution has emerged as a formidable challenge, prompting a pressing need for solutions. The utilization of metal nanoparticles with surface plasmon resonance and semiconductor composite photocatalysts is regarded as a highly effective approach to solve this problem. gC3N4 is an effective catalyst for the degradation of organic pollutants. Its photocatalytic performance could be enhanced by the use of the metal such as Au, Ag, and Cu. However, the Schottky barrier is generally believed to be the reason that Cu can enhances the photocatalytic performance of gC3N4. In fact, the plasmonic of Cu will also have an effect on the photocatalytic effect of gC3N4, like Au and Ag. In this study, it is first proposed that the photocatalytic performance of gC3N4 can be enhanced by combining the plasmonic of Cu and the Schottky barrier. Cug/C3N4 nanocomposites with stable crystal structure, with Cu as the core and gC3N4 as surface coating, were synthesized and characterized by transmission electron microscopy (TEM), Xray diffractometer (XRD) and ultravioletvisible spectroscopy (UVVis) and Xray photoelectron spectroscopy (XPS). Then, it is optimized that the Cu/gC3N4 composite material which were prepared with 200 nm Cu particles, GSH as the ligand bridge, and a Cu: gC3N4 ratio of 5:1, have the best photocatalytic property, which is 16.7 times of the photocatalytic property of gC3N4. Besides, the transfer efficiency of hot electrons of 33% (from Cu to gC3N4) under the influence of the Schottky barrier (0.58 eV) is calculated theoretically. On this basis, the mechanism of Cu nanoparticles enhancing the photocatalytic performance of gC3N4 was also been proved through the calculation result and transient photocurrent spectroscopy (TPC), electrochemical impedance spectroscopy (EIS), and steadystate transient fluorescence spectroscopy (PL) characterization. The excellent photocatalytic performance of the Cu/gC3N4 composites is due to the simultaneous influence of Schottky barrier and LSPR of Cu.

G-C3N4 modified with non-precious metal Al with LSPR as an efficient visible light catalyst.

The issue of water pollution has emerged as a formidable challenge, prompting a pressing need for solutions. The utilization of metal nanoparticles with surface plasmon resonance and semiconductor composite photocatalysts is regarded as a highly effective approach to solve this problem. g-C3N4 is an effective catalyst for the degradation of organic pollutants. Its photocatalytic performance is usually enhanced by the use of the noble metal Au Ag. However, the high cost of these materials limits their application. In this study, we present the synthesis of Al NPs/g-C3N4 nanocomposites using the bridging effect of ligands. The characterized of transmission electron microscopy (TEM), X-ray diffractometer (XRD) and ultraviolet-visible spectroscopy (UV-Vis) proved that Al NPs/g-C3N4 with a wider light absorption range were successfully synthesized. The effects of ligands, (glutathione (GSH), glutamic acid (GAG), and cysteine (CYS)), Al diameter (40 to 200 nm) and the ratio of Al to g-C3N4 (1:1 to 5:1) on the photocatalytic degradation of methylene blue (MB) by Al NPs/g-C3N4 were also evaluated. The results showed that the optimum degradation efficiency of Al NPs/g-C3N4 for MB at 5 mg/L reached 100% within 60 min, which was 11 times higher than that of pure g-C3N4. The principal analysis of Al enhancing the photocatalytic performance of g-C3N4 was studied through transient photocurrent spectroscopy (TPC), electrochemical impedance spectroscopy (EIS), and steady-state transient fluorescence spectroscopy (PL). The results confirmed that hot carriers generated by localized surface plasmon resonance (LSPR) of Al nanoparticles increase the carrier concentration. In addition, the Schottky barrier generated by Al and g-C3N4 could also improve the carrier separation rate and increase the carrier lifetime. This work is expected to solve the problem of organic wastewater treatment and lay the foundation for subsequent research on photocatalysis.

Promotion effect of Ga3+ in ZnAlGa-x mixed oxides obtained from their layered double hydroxides on the enhancement in the transient photocurrent response, the inhibition in the recombination rate of photogenerated (e−, h+) charges and the photocatalytic activity in the photodegradation of 4-chlorophenol

ZnAlGa-x (x = 0, 0.5, 0.8, 1.0, 2.0 and 3.0 mol % of Ga3+) mixed oxides (MOs) obtained by calcination of their corresponding layered double hydroxides (LDHs) were characterized and evaluated in the photodegradation of 4-chlorophenol in aqueous media (4-CP) using UV light irradiation. Structural studies by X-ray diffraction (XRD) of ZnAlGa-x MOs indicated that the incorporation of a small molar concentration of Ga3+ (0.5–0.8 mol %) within the ZnAl LDH structure changed the micro-crystalline structure to an amorphous structure. A small molar content of Ga3+ in the ZnAlGa3+ MOx promotes the formation of a high amount of structural oxygen vacancies as evidenced by Raman and EPR spectroscopy studies. Transient photocurrent response (TPC) and fluorescence spectroscopy experiments reveal that the incorporation of a small molar concentration of Ga3+ in the structure of ZnAl MO promotes the enhancement in the transient photocurrent response, the separation of photogenerated charges and the inhibition of the recombination rate of (e−, h+) pairs. Furthermore, ZnAlGa-x MOs with a small molar concentration of Ga3+ reduced their band gap energies below of 3.0 eV. ZnAl MO photodegradated 69.0 % of 4-CP after 4 h of reaction. However, the photoconversion % of 4-CP increased to 80–90 % at 3 h of reaction using ZnAlGa-x MOs with a small molar concentration of Ga3+. ZnAlGa-x with small Ga3+ concentration mineralized approximately 70–75 % of total disolved organic carbon at 3 h of reaction. The photocatalytic degradation reaction of 4-CP fitted well to a pseudo-first order kinetics of Langmuir-Hinshelwood-type. The kinetic parameters of ZnAlGa-x MOs confirmed that ZnAlGa-0.8 MO was the best photocatalyst to photodegradate 4-CP in aqueous media. The highest photocatalytic activity and efficacy of ZnAlGa-0.8 MO to mineralize 4-CP in aqueous medium may be due to its higher amount of oxygen vacancies contained in its structure promoting a better separation of charges and the inhibition of the recombination rate of photogenerated (e−, h+) charges.

Classification of Dairy Products by Time-Correlated Single Photon Counting Fluorescence and Principal Component Analysis (PCA)

A transient fluorescence spectroscopy system based on time-correlated single photon counting (TCSPC) technology was constructed. Time-resolved fluorescence spectra of pure, modified and fresh milk were obtained using this system. A method for differentiating these dairy products was established by principal component analysis (PCA). The measured time-resolved fluorescence spectra of the dairy products were directly substituted into the principal component analysis model without fitting. The results showed that the two largest principal components, PC1 and PC2, had variance contribution values of 48.18% and 32.1%, respectively. The results were plotted as a two-dimensional graph of PC1 × PC2. The results showed that the coordinates of the dairy products were clearly distinguished in the two-dimensional plot, with pure milk in the fourth quadrant, modified milk in the second and third quadrants, and fresh milk in the first quadrant. It is inferred that the time-resolved fluorescence spectra were changed due to the different heat treatment of the dairy products. Compared with fluorescence spectroscopy, time-resolved fluorescence spectroscopy is less susceptible to instrumental variation. Data from different instruments can be compared. The developed method does not require any processing and fitting of fluorescence lifetimes to milk, which has the advantages of easy identification, simple operation, and short identification time.

Isomeric fluorescence sensors for wide range detection of ionizing radiations

In order to achieve a wider range of ionizing radiations detection, novel fluorescence sensing materials have been developed that utilize the fluorescence enhancement phenomenon caused by the intramolecular photoinduced electron transfer (PET) effect. Two perylene diimide isomers PDI-P and PDI-B were designed and synthesized, and their molecular structures were characterized by high-resolution Fourier transform mass spectrometry (HRMS), nuclear magnetic resonance hydrogen and carbon spectroscopy (1H and 13C NMR). The interaction between ionizing radiation and fluorescent molecules was simulated by HCl titration. The results show that combining PDIs and HCl can improve fluorescence through the retro-PET process. Despite the similarities in chemical structures, the fluorescent enhancement multiple of PDI-B with aromatic amine as electron donor is much higher than that of PDI-P with alkyl amine. In the direct irradiation experiments of ionizing radiation, the emission enhancement multiples of PDI-P and PDI-B are 2.01 and 45.4, respectively. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations indicate that the HOMO and HOMO-1 energy ranges of PDI-P and PDI-B are 0.54 eV and 1.13 eV, respectively. A wider energy range has a stronger driving force on electrons, which is conducive to fluorescence quenching. Both femtosecond transient absorption spectroscopy (fs-TAS) and transient fluorescence spectroscopy (TFS) tests show that PDI-B has shorter charge separation lifetime and higher electron transfer rate constant. Although both isomers can significantly reduce LOD during PET process, PDI-B with aromatic amine has a wider detection range of 0.118–240 Gy due to its larger emission enhancement, which is a leap of three orders of magnitude. It breaks through the detection range of gamma radiation reported in existing studies, and provides theoretical support for the further study of sensitive and effective new materials for ionizing radiation detection.

Construction of a Fe2O3/Ti3C2Tx heterojunction photocatalyst with oxygen-rich vacancies and its photocatalytic properties

As a class of antibiotics, tetracycline (TC) poses a serious threat to aquatic environments, thereby necessitating new demands on their restoration. Herein, a Fe2O3/Ti3C2Tx heterojunction photocatalyst with abundant surface oxygen vacancies was prepared using a solvothermal method, which can promote the migration and separation of photogenerated electron–hole pairs and improve the catalytic degradation performance of the material. The experimental results revealed that the prepared Fe2O3/Ti3C2Tx heterojunction catalysts exhibited better TC degradation performance under simulated sunlight, corresponding to 2.5 times that of Fe2O3 and 5.1 times that of Ti3C2Tx. Fe2O3 and Ti3C2Tx formed a close contact interface, and the staggered energy-level structure achieved the rapid transfer of electrons, resulting in the formation of numerous oxygen vacancies during the conversion of Fe3+ to Fe2+; these oxygen vacancies not only changed the Fe2O3/Ti3C2Tx crystal structure but also provided additional active centers for the reaction. In addition, transient fluorescence spectroscopy and photocurrent experiments revealed that the method considerably improved the separation rate of photo‐excited carriers and the corresponding optoelectronic capability. The possible degradation pathways were revealed by the determination of the TC post-degradation by-products. Moreover, toxicity modeling results revealed that most degradation intermediates were less toxic than TC. This study discusses the importance of heterojunctions with oxygen-rich vacancies for improving the performance of degraded TC and provides an effective strategy for constructing heterojunction photocatalysts, broadening the applications of Fe2O3 and Ti3C2Tx materials in environmental remediation.

Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design.

The complex photokinetics of donor-acceptor-donor triads with varying flexible spacer lengths (n = 4-10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady-state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in liquid solution, dynamic charge transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows to reassign the observations of a former seminal report on stimuli-responsive high-contrast fluorescence on/off switching in films of the triads (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in-depth analysis of complex photophysics can be put to practical use in materials science. This article is protected by copyright. All rights reserved.

Cryo-EM and femtosecond spectroscopic studies provide mechanistic insight into the energy transfer in CpcL-phycobilisomes

Phycobilisomes (PBS) are the major light harvesting complexes of photosynthesis in the cyanobacteria and red algae. CpcL-PBS is a type of small PBS in cyanobacteria that transfers energy directly to photosystem I without the core structure. Here we report the cryo-EM structure of the CpcL-PBS from the cyanobacterium Synechocystis sp. PCC 6803 at 2.6-Å resolution. The structure shows the CpcD domain of ferredoxin: NADP+ oxidoreductase is located at the distal end of CpcL-PBS, responsible for its attachment to PBS. With the evidence of ultrafast transient absorption and fluorescence spectroscopy, the roles of individual bilins in energy transfer are revealed. The bilin 1Iβ822 located near photosystem I has an enhanced planarity and is the red-bilin responsible for the direct energy transfer to photosystem I.

In situ construction of rich oxygen vacancy Bi/Bi3TaO7 heterojunction photocatalysts

Oxygen vacancy-rich Bi/Bi3TaO7 binary nanomaterials were obtained by a simple in situ reduction. The as-prepared composite exhibited superior photocatalytic activity for the degradation of RhB compared to the single material under the visible light range. The degradation rate of the optimal sample reached 92% within 120 min, 12.25 times that of the pure Bi3TaO7. XRD and HRTEM characterization showed that the Bi metal obtained through in situ reduction was loaded on the Bi3TaO7 nanostructure. In addition, XPS and EPR indicated a significant increase in the oxygen vacancy content of the composites. The time-resolved transient fluorescence spectroscopy and photocurrent results proved that the separation rate of photoexcited carriers and the corresponding ability of photoelectrons are greatly increased. It is due to the synergistic effect of the unique photo-reaction excitation pathway induced by bismuth metal plasma resonance and oxygen vacancies, which not only extends the photo-reaction to the infrared region but also increases the carrier residence time, and this is the fundamental reason for the excellent photocatalytic performance of the composite material. The h+ is the main reactive radical of the reaction, and •O2− also plays a role. Based on the above characterization study, we propose a possible photoreaction mechanism for the synergistic oxygen vacancy and Bi metal of Bi/Bi3TaO7.

Rational Control of Near-Infrared Colloidal Thick-Shell Eco-Friendly Quantum Dots for Solar Energy Conversion.

Thick-shell colloidal quantum dots (QDs) are promising building blocks for solar technologies due to their size/composition/shape-tunable properties. However, most well-performed thick-shell QDs suffer from frequent use of toxic metal elements including Pb and Cd, and inadequate light absorption in the visible and near-infrared (NIR) region due to the wide bandgap of the shell. In this work, eco-friendly AgInSe2 /AgInS2 core/shell QDs, which are optically active in the NIR region and are suitable candidates to fabricate devices for solar energy conversion, are developed. Direct synthesis suffers from simultaneously controlling the reactivity of multiple precursors, instead, a template-assisted cation exchange method is used. By modulating the monolayer growth of template QDs, gradient AgInSeS shell layers are incorporated into AgInSe2 /AgInS2 QDs. The resulting AgInSe2 /AgInSeS/AgInS2 exhibits better charge transfer than AgInSe2 /AgInS2 due to their favorable electronic band alignment, as predicted by first-principle calculations and confirmed by transient fluorescence spectroscopy. The photoelectrochemical cells fabricated with AgInSe2 /AgInSeS/AgInS2 QDs present ≈1.5-fold higher current density and better stability compared to AgInSe2 /AgInS2 . The findings define a promising approach toward multinary QDs and pave the way for engineering the QDs' electronic band structures for solar-energy conversion.

Effective fluorescent turn-on detection of ionizing radiations through controllable intramolecular photoinduced electron transfer

In order to explore the detection of gamma radiation via intramolecular photoinduced electron transfer (PET) and the interaction between molecular structures and fluorescence enhancement, we conducted an in-depth study of the perylene tetracarboxylic diimide (PDI) series of fluorescent probes capable of detecting HCl in the non-polar CHCl3 solutions. To simulate the interaction between ionizing radiation and fluorescent molecules, we used HCl titration method to predict the ability of PDI-n to detect gamma rays. The results show that the combination of PDI-n and HCl can enhance the fluorescence through retro-PET process. In addition, the fluorescence enhancement multiple gradually decreased with the enlargement of alkylamine side chains. In the direct irradiation experiment of ionizing radiation, PDI-n can also show a good fluoresence enhancement to gamma rays. Similar to the above HCl titration tests, the fluorescence increament stronthly depends on the alkylamine side chains lengths. The maximum fluorescence increament of PDI-1 is 22.24, and the fluorescence enhancement multiple of PDI-1–PDI-4 decreases when increasing the alkyl chain length. Besides, combining density functional theory (DFT), time-dependent density functional theory (TDDFT) calculations and electrochemical tests, the relative positions of each energy level obtained from the experiment indicate that the PDI-n molecule is beneficial to the occurrence of the intramolecular PET effect. Using femtosecond transient absorption spectroscopy (fs-TAS) and transient fluorescence spectroscopy (TFS) tests, the lifetime of charge separation was obtained, and the efficiency of the PET process was characterized by the rate constant. We found that fluorescent molecules with shorter side chains can realize PET more efficiently. Therefore, this work exploits the intramolecular PET mechanism for efficiently detecting gamma rays using a series of fluorescent probes, and reveals the law of the influence of molecular chain length on the PET effect. Which provides theoretical support for the development of new materials for effective and sensitive detection of ionizing radiation.

On the Molecular Origin of the Red Emission in the Newly Synthesized Carbon‐Based Quantum Dots

AbstractCarbon‐based quantum dots (QDs) represent a new family of luminescent nanomaterials with intriguing emission properties. They are synthesized predominately with the emission in the blue and green region of the visible spectrum, with limited success in producing red emission. Furthermore, the current literature on the subject lacks consensus with respect to the morphology of QDs. Contrary to their semiconductor counterparts, the data on the structure of fluorescing centers is scarce. Herein, we describe a facile one‐pot synthesis of red emissive QDs using citric acid and formamide as precursors without addition of any other source of nitrogen. The photophysical properties of the synthesized species were investigated by steady state and transient absorption and fluorescence spectroscopy. Some structural peculiarities were revealed using fluorescence correlation spectroscopy. Our findings show that the newly synthesized carbon‐based QDs, at least their emissive centers, are not part of nanoparticles but rather resemble small organic fluorophores. Furthermore, we have not eliminated the possibility that multiple fluorophores with different emission properties may be embedded within a single molecular entity.

Enhanced luminescence efficiency and thermal stability via introduction of non-rare earth Bi3+ in Gd5Si2BO13:Eu3+

A series of Gd5Si2BO13:Eu3+ and non-rare earth Bi3+ ions doped Gd5Si2BO13:Eu3+ phosphors was successfully synthesized via high-temperature solid-state method, and the as-obtained phosphors were studied on their phase structures, luminescence characteristics, thermal stability and luminescence lifetime. Transient fluorescence spectroscopy data show that the addition of Bi3+ can obviously enhance the emission intensity of Eu3+ in the near-ultraviolet band owing to energy transfer from Bi3+ to Eu3+. Besides, the addition of Bi3+ can improve the thermal stability of a single-doped phosphor Gd5Si2BO13:0.35Eu3+ from 70.33% to 87.45% at 150 °C and the quantum yield from 58.80% to 78.61% at room temperature. Finally, Gd5Si2BO13:Eu3+,Bi3+ was used to encapsulate white light-emitting diodes (WLEDs) with green ((Ba,Sr)2SiO4:Eu2+) and blue (BaMgAl10O17:Eu2+) commercial phosphors. The color rendering index of WLEDs was calculated to be larger than 90, and the color temperature was estimated to be 4300–4500 K, which demonstrate that Gd5Si2BO13:Eu3+,Bi3+ can be regarded as a red phosphor with great potential application. This paper can provide a new insight into design of high-efficiency phosphors by introducing non-rare earth Bi3+ ions via energy transfer from Bi3+ to Eu3+.

Determining the Energy Gap between the S1 and T1 States of Thermally Activated Delayed Fluorescence Molecular Systems Using Transient Fluorescence Spectroscopy.

The energy gap (ΔES-T) between the lowest single and triple excited states is a crucial parameter for thermally activated delayed fluorescence (TADF) molecular systems with high quantum yield. However, a reliable experimental approach to precisely determine this value is challenging. Here, we introduce a new, simple, and efficient strategy to accurately obtain the ΔES-T in TADF systems from time-resolved fluorescence spectroscopy using a recently reported TADF molecule, DMACPDO, as a representative. By introducing an explicit model to describe the corresponding singlet-triplet coupling system, elusive intersystem crossing and reverse intersystem crossing rates can be extracted by fitting the kinetics of the observed fluorescence. The ΔES-T value can then be determined. Moreover, our modeling accurately explained the opposite trend in fluorescence intensity of DMACPDO with solvent polarity under air-saturated and deoxygenated conditions. Additionally, the validity of this approach has been demonstrated in another well-known TADF molecule, 4CzIPN. We demonstrate how this approach of determining ΔES-T sheds light on a deeper understanding of energy-loss mechanisms involved in related photoconversion processes.

Interactions of cationic gold nanoclusters with serum proteins and effects on their cellular responses

Cationic nanoparticles (NPs) have shown great potential in biological applications owing to their distinct features such as favorable cellular internalization and easy binding to biomolecules. However, our current knowledge of cationic NPs’ biological behavior, i.e., NP–protein interactions, is still rather limited. Herein, we choose ultrasmall-sized fluorescent gold nanoclusters (AuNCs) coated by (11-mercaptoundecyl) - N, N, N - trimethylammonium bromide (MUTAB) as representative cationic NPs, and systematically study their interactions with different serum proteins at nano-bio interfaces. By monitoring the fluorescence intensity of MUTAB-AuNCs, all proteins are observed to bind with roughly micromolar affinities to AuNCs and quench their fluorescence. Transient fluorescence spectroscopy, X-ray photoelectron spectroscopy and isothermal titration calorimetry are also adopted to characterize the physicochemical properties of MUTAB-AuNCs after the protein adsorption. Concomitantly, circular dichroism spectroscopy reveals that cationic AuNCs can exert protein-dependent conformational changes of these serum proteins. Moreover, protein adsorption onto cationic AuNCs can significantly influence their cellular responses such as cytotoxicity and uptake efficiency. These results provide important knowledge towards understanding the biological behaviors of cationic nanoparticles, which will be helpful in further designing and utilizing them for safe and efficient biomedical applications.

Fluorescent iron‑sulfur centers: Photochemistry of the PetA Rieske protein from Aquifex aeolicus

Cytochrome bc1 complexes are energy-transducing enzymes and key components of respiratory electron chains. They contain Rieske 2Fe2S proteins that absorb very weakly in the visible absorption region compared to the heme cofactors of the cytochromes, but are known to yield photoproducts. Here, the photoreactions of isolated Rieske proteins from the hyperthermophilic bacterium Aquifex aeolicus are studied in two redox states using ultrafast transient fluorescence and absorption spectroscopy. We provide evidence, for the first time in iron‑sulfur proteins, of very weak fluorescence of the excited state, in the oxidized as well as the reduced state. The excited states of the oxidized and reduced forms decay in 1.5 ps and 30 ps, respectively. In both cases they give rise to product states with lifetimes beyond 1 ns, reflecting photo-reduction of oxidized centers as well as photo-oxidation of reduced centers. Potential reaction partners are discussed and studied using site-directed mutagenesis. For the reduced state, a nearby disulfide bridge is suggested as an electron acceptor. The resulting photoproducts in either state may play a role in photoactivation processes.

Synthesis and characterization of a twisted tri-sulfur-doped polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAHs) generally refer to aromatic compounds containing two or more fused benzene rings. PAHs have excellent optical, electronic and self-assembly properties, and have important applications in organic light-emitting diodes (OLED), organic field effect transistors (OFET), organic solar photovoltaic cells (OPVC) and other fields. The introduction of heteroatoms to PAHs can adjust the electronic structure, dipole moment, and the self-assembly behavior. Among them, sulfur-doped PAHs have been widely studied. Due to the proper energy level and bonding mode of sulfur atom, the sulfur-doped PAHs have stronger dipole-dipole interactions, electron donating ability, and weakened aromaticity. Sulfur-doped PAHs usually exhibit unique redox characteristics and better hole transporting, so they are widely used as p-type organic semiconductor and electronic donor in photoelectric conversion, and donation unit in donor-acceptor conjugated molecules. Doping PAHs with sulfur is mainly through two ways: post-modification and fusing the sulfur-containing heterocyclic unit. On the other hand, regarding the regulation of geometric structure of carbon skeleton, non-hexagonal ring is introduced to transform the molecular structure from planar to curved structure. The structural distortion can be achieved by the peripherial stereo-hindrance. For example, the PAHs containing helicene units have a highly distorted structure, thus showing a unique optical, electric and assembly properties. In order to construct novel PAHs, this article combines heteroatom doping and geometric control of carbon skeleton. A twisted tri-sulfur-doped PAH C36S3H10(C9H11)6 (molecule 1 ) was synthesized by oxidative cyclodehydrogenation of an oligophenylene precursor, which contains three thiophene rings. X-ray single crystal diffraction exactly characterized the structure of molecule 1 . Because the ring is not completely fused, molecule 1 contains two [5]helicene units, which leads to a twisted molecular structure. In crystalline state, molecule 1 forms dimers through C−H···π and weak π-π interactions, which further packs into one-dimensional columnar stacking through the weak interactions of C−H···π and C−H···S. Different from the planar PAHs, the twisted molecule 1 is not easy to aggregate in solution. Molecule 1 exhibits a yellow photoluminescence, but its quantum yield is low. Combined with the study of transient fluorescence spectroscopy, it is speculated that its twisted molecular structure may be the reason for its low quantum yield. The synthesis of molecule 1 enriches the family of sulfur-doped PAHs, and the derivatives with alkyl chains of molecule 1 can be synthesized in the future and be used as organic semiconductors or electron donors in solar cells and organic electronic devices, etc.

Thermally Stable Phosphor KBa2(PO3)5:Eu2+ with Broad-Band Cyan Emission Caused by Multisite Occupancy of Eu2.

The relationship between the structure and properties is always a hot topic in the luminescent material field. In this work, a new phosphor KBa2(PO3)5:Eu2+ (KBP:Eu) was prepared by a high-temperature solid-state reaction method and characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, photoluminescence (PL), and electroluminescence studies. The polyphosphate host KBP offers three lattice environments (K1, Ba1, and Ba2) for Eu2+ ions to realize broad-band emission from 380 to 700 nm under 345 nm excitation. The distributions of Eu2+ in the three lattice sites can be proven by low-temperature PL and transient fluorescence spectroscopy. Furthermore, temperature-dependent luminescence studies for phosphor KBP:0.02Eu reveal that its luminescence intensity at 150 °C retains about 97% of the initial value at 25 °C. By composing a 365 nm UV chip and KBP:0.02Eu, CaAlSiN3:Eu2+ phosphors, a warm white-light-emitting diode (WLED) was obtained with a correlated color temperature of 5146 K and chromaticity coordinates (0.3404, 0.3384). Therefore, KBP:Eu phosphor is a potential cyan-emitting phosphor used for high-power WLEDs.